All the living cells that make up our body and the body of a spider need a constant supply
of oxygen to survive and this is pumped by a heart through blood vessels. In the case of
people, a special protein called haemoglobin, which is stored inside red blood cells,
carries oxygen molecules. Our blood is red because the haemoglobin molecule contains
oxygenated iron. Spider blood or haemolymph, is blue because the oxygen-carrying
protein haemocyanin contains oxygenated copper. Haemocyanin is not stored within
blood cells, but instead floats freely in the haemolymph.

Spiders have a very simple heart, which is in their abdomen, and is little more than
a tube with a couple of valves to make sure the blood flows in one direction.
When muscles attached to the heart contract, the tube enlarges and it fills with fluid.
The muscles then relax and blood is forced out of the heart and around the body
of the spider. The heart beat generated by the contraction and relaxation of
muscles varies depending on what the spider is doing. For example, a wolf
spider has a resting heart beat of about 50 beats a minute, but this will increase
to about a 180 beats per minute if it is running.

With a microscope, you can easily see the heart beating beneath the thin wall of
the abdomen in light-coloured spiders. And a racing heart will be a tell-tale
sign that the spider would like to be running away!